Comparative study on the compressive performance of honeycomb structures fabricated by stereo lithography apparatus

2.1 Materials

Three raw materials were used in the experiment, with the trade names being photosensitive Resin DSM 8000, photosensitive Resin Somos^® Taurus, and Nylon FS 3400GF. Photosensitive Resin DSM 8000 has stable viscosity, efficient molding, excellent detail resolution, and high durability. This material is particularly suitable for functional prototypes, conceptual models, and small batch production components, which have durability and moisture resistance [25]. Photosensitive Resin Somos^® Taurus is an extremely tough and high-temperature resistant three-dimensional photocurable printing material, combined with excellent durability and black appearance, making it a versatile prototype and final product application. In Nylon FS 3400GF, glass microspheres account for 40%. Nylon is composed of glass microspheres, which have high stiffness, low relative density, high thermal deformation temperature, and excellent strength and hardness. It is suitable for products with high strength requirements, as well as parts that require their own weight, aerospace parts, and pneumatic components in automotive sports applications. The physical properties of the three raw materials are shown in Table 1.

2.2 Preparation of honeycomb structural components

The 3D printing of honeycomb structural components as designed in Figure 1(a) adopts the D300 SLA printing equipment produced by Shanghai UnionTech Technology Co., Ltd, as shown in Figure 1(b), with a molding size of 300 × 300 × 100 mm³ and an accuracy of 50–100 µm. The laser is a solid-state triple frequency Nd:YVO₄ running at a typical scanning speed of 6–10 m/s, and UnionTech RSCON operating software, rated power 2.6 kVA. The forming process is mainly divided into three stages: preparation before printing, printing forming stage, and post processing stage. First, prepare the model and design a 3D model of honeycomb structural components by using 3D CAD software as shown in Figure 1(a). Second, during the printing and forming stage, set the printing parameters that meet the requirements of the part forming. The printing size parameters for the three materials are listed in Table 2, and the part prototype is printed. Then, rinse with industrial alcohol at a concentration of over 95% and perform post-curing treatment to obtain a three-dimensional honeycomb structural component, as shown in Figure 1(c). After printing, measure the size of the actual component as shown in Table 2.

Figure 1

Printing honeycomb structural components: (a) 3D data model, (b) D300 SLA printing equipment, (c) as-made honeycomb structural components of three materials, (d) vertical compression, and (e) transverse compression.

2.3 Testing and characterization

The Hitachi S3400N scanning electron microscope (SEM) was used to observe the micro-surface morphology, and a multifunctional fast rotating confocal microscope Smartproof 5 (ZEISS, Germany) was used for surface analysis to measure the surface roughness values of 3D printed structural parts. The surface analysis was carried out at the following conditions, such as a wavelength of 405 nm, a resolution of 120 nm, 20 nm for Z-axis minimum step accuracy, up to 5 mm for the height scanning range, with the objective lens of 2.5× ∼50×, and the stage dimension of 150 mm × 150 mm super large stroke. The compression experiments were conducted via Material Test System testing machine on the sample in both horizontal and vertical directions with a maximum load of 100 kN and a speed of 1 mm/min. During the mechanical test, Figure 1(d) represents vertical compression and Figure 1(e) represents transverse compression. In addition, a digital microhardness tester was used for surface mechanical performance analysis with a loading force of 10 gf and a holding time of 10 s.

3.1 Surface morphology analysis

SEM was used to observe the micro-surface morphology of the formed honeycomb structure samples as demonstrated in Figure 2, and it was found that the surface morphology on the honeycomb structures made of the three materials had their own unique characteristics, which is possible due to the nature of each one’s response to the light irradiation. Among them, the white surface of DSM 8000 is smooth, and it also exhibits obvious distribution of oriented bands or ridges at a regular interval as depicted by the red dashed line in Figure 2(a), which is closely related to the color of the material. UV light shining on the surface of the white prepolymerized monomer will generate reflection [29] triggering a chain reaction of similar photo-initiators. And the black surface of Somos^® Taurus exhibits a uniform and regular lattice shape as the red box in Figure 2b, where UV light on the surface of the black prepolymerized monomer is fully absorbed with little energy leakage [30]. Therefore, the lattice size on the surface of this material is most likely to be determined by the size of the illuminated spot. Unlike the two as mentioned above, there are irregularly shaped microspheres on the surface of the structural components made of FS 3400GF, and that coarse surface could be ascribed to content of containing 40% glass microspheres in this material, which could be due to gap in the melting points of glass microspheres and nylon causing noticeable cooling shrinkage difference between the two ingredients from liquid mixture to solid status. Such observations remind of paying attention on some critical parameter in possible materials with different formulas using SLA technique, e.g., thermal property differences.

Figure 2

Surface SEM images of three types of honeycomb structural components magnified by 50 times of (a) DSM 8000, (b) Somos^® Taurus, and (c) FS 3400GF; magnified by 200 times of (d) DSM 8000, (e) Somos^® Taurus, and (f) FS 3400GF.

Furthermore, it can be observed in Figure 2(d) that the surface of material DSM 8000 at a magnification of 200× contains granular impurities with different sizes, which might be because of some impurities in the raw materials. Some solid particles stand out on the surface of the component, while others stay underneath the surface and inside the component. While the detailed surface morphology in microscale as demonstrated in both Figure 2(e) and (f) state otherwise significantly. In Figure 2(e), it can be observed that the grids are stacked together in a regular manner, and bands appear within each grid unit, which is caused by the equidistant grating generated by UV light passing through the laser crystal. In Figure 2(f), it can be observed that holes have appeared inside and on the surface of the structural component, which is due to some of the light being blocked or refracted by glass microspheres during UV irradiation, resulting in uneven energy gain from the initiator.

3.2 Surface roughness analysis

Surface roughness morphology is one of the most important parameters for evaluating machining quality [31]. The surface roughness index parameters of honeycomb structural components made of three materials are shown in Figure 3(a) and (b). Among them, the arithmetic mean height (S _a) is the average of the absolute values of the height differences between various points on the surface, which is generally used to evaluate surface roughness [32]. The root mean square height (S _q) is the root mean square of the height of each point in the swept area, equivalent to the standard deviation of the height. S _p and S _v are the maximum peak height and maximum valley depth in the swept area, respectively. The maximum height (S _z ) is the sum of the maximum peak height (S _p) and the maximum valley depth (S _v). In addition, skewness (S _sk) is the degree of deviation of surface concavity and convexity, which can be used to determine the parameter of roughness shape (concavity and convexity) tendency through the numerical value of S _sk. Kurtosis (S _ku), also known as sharpness, can be used to determine the parameters of roughness, shape, and sharpness. The functional parameter (S _mc) is the inverse ratio of materials in the same region, indicating the reverse loading area rate, S _mc(p) represents the height that meets the loading area rate p%.

Figure 3

Surface roughness evaluation of three types of honeycomb structural components: (a) plot of surface roughness parameters, (b) Skewness (S _sk) and kurtosis (S _ku), 3D surfaces roughness view of (c) DSM 8000, (d) Somos^® Taurus, and (e) FS 3400GF.

As can be seen from Figure 3(a) that using the S _a value to evaluate the surface roughness index, the surface roughness of structural components of the three materials is similar, and there is no significant difference. The results of S _p and S _v indicate that DSM 8000 has the highest value, followed by FS 3400GF and Somos^® Taurus. Therefore, the surface roughness indicators of Somos^® Taurus honeycomb structural components are lower than the other two.

Second, the results of S _sk indicate that the height distribution of the three structural components is relatively lower than the average surface, showing a valley. Among them, the difference in surface roughness wave valleys between DSM 8000 and FS 3400GF structural components is small, while the surface roughness wave valleys of Somos^® Taurus honeycomb structural components are the most significant in depth. Relatively speaking, the S _ku results indicate the presence of obvious needle like sharp spikes on the surfaces of all three, and that of DSM 8000 is being the most prominent, FS 3400GF material taking the second place, and Somos^® Taurus honeycomb structural components being the least significant.

In addition, the roughness morphology image as portrayed in Figure 3(c) further confirms the existence of millimeter-scaled bands and ridges along with obviously same direction distribution on the surface of the DSM 8000 structural plane (S _z = 1,060 µm, S _p = 575 µm), which are twice that of the Somos^® Taurus honeycomb structural component as shown in Figure 3(d), and the latter is mostly uniformly distributed with gradual bands and grooves. The results indicate that the surface morphology of structural components made of DSM 8000 and FS 3400GF materials is irregular and scattered, with the smallest degree of surface roughness deviation (S _sk = 9.17%). The surface of Somos^® Taurus as shown in Figure 3(d) exhibits a uniform and regular lattice pattern. The surface of the structural components of FS 3400GF material as shown in Figure 3(e) exhibits irregular shape and size distribution of micro-bead particles and their aggregates, so there are no obvious striped ridges or concave convex strips on the surface.

3.3 Compressive mechanical behavior analysis

The compressive mechanical behavior analysis of honeycomb structural components made of three materials is trying to figure out how the materials and structure contributed to the overall compression response. And the vertical (V) compression mechanical behavior of honeycomb structural components is plotted in Figure 4(a). It is very noticeable that FS 3400GF can withstand a maximum force of 14.1 kN in vertical compression, while DSM 8000 can hold up to a relative less maximum force of 10.1 kN, and Somos^® Taurus is the lowest one of 5.8 kN. The vertical compression response indicated that FS 3400GF with such honeycomb structure bears significantly higher pressure than the other two materials, due to the presence of glass microspheres as reinforcing filler in FS 3400GF [33]. As the vertical pressure gradually increases further, the honeycomb structure plays a primary role, followed by the overall deformation of the structure, which makes the mechanical properties of the material to play a secondary role. Finally, the inconsistent deformation of the stress surface of the honeycomb structure results in different failure modes, indicating that this is closely related to the printed honeycomb structure, but has a secondary correlation with the materials themselves and their composition. This can also be consistent with the macroscopic failure morphology presented in the V direction as depicted by the failure observation shown in Figure 5.

Figure 4

Compression force curves of three types of honeycomb structural components in (a) vertical and (b) transverse directions.

Figure 5

Failure patterns of three types of honeycomb structural components before and after compression test. (a) DSM 8000, (b) Somos^® Taurus, and (c) FS 3400GF (O represents uncompressed state, V represents vertical, and H represents transverse, the red arrow represents the direction of force application and the blue circle represents the location of transverse failure).

Compared with the vertical direction, the transverse (H) compression behavior is shown in Figure 4(b). As different to that of vertical one, FS 3400GF could bear a maximum lateral force of 0.47 kN, DSM 8000 is of 0.56 kN, and Somos^® Taurus is of 0.17 kN. Such significant differences indicate that lateral compression failure is highly likely to occur, which is more distinct from vertical compression failure. This also implies that the magnitude of compressive force that the structure can withstand in shoulder-to-shoulder combinations similar to that of regular hexagons is correlated with material properties, while the testing condition has little effect on the structure. This conclusion is completely consistent with the macroscopic morphology characteristics of cracks and deformation failure presented in the H-direction compression test in Figure 5. The maximum force and displacement endured are entirely determined by the mechanical properties of the specific material itself.

In Figure 5, the vertical failure is manifested as failure on the contact plane, with the surface depressions on DSM 8000 and FS 3400GF, while Somos^® Taurus has cracks near the contact surface. The transverse compression failure mode starts from the inside, such as Somos^® Taurus and FS 3400GF structural components lack an internal piece and have no obvious surface damage, while the DSM 8000 is the most severely damaged, directly splitting into two parts.